Electric hygrometer



Jan. 16, 1945 G. BoUYoUcos ELECTRIC HYGROMETER Filed April 1o, 1945 Patented Jan. 16, 1945 ELECTRIC HYGROMETER George Bouyoucos, East Lansing, Mich., assignor to Michigan State Board ol' Agriculture. East Lansing, Mich.,.a corporation of Michigan Application April 10, 1943, Serial No. 482,564

8 Claims.

This invention relates to electrical apparatus for measuring the relative humidity of an atmosphere containing moisture, and more particularly to a novel form of moisture absorbing resistor for use therewith.

At the present time there are only three types of apparatus commonly employed for measuring relative humidity of air. They are the wet and dry bulb thermometers, the hair hygrometer, and apparatus for determining dew point. None of these devices is satisfactory, yet they have been in practically exclusive use for nearly a century because no superior apparatus has been produced.

The wet and dry bulb thermometers are inconvenient to use because they must be rapidly moved about in the atmosphere being measured. Furthermore, they are quite inaccurate at very high humidities and are entirely worthless for recording the lowest humidities.

The hair hygrometer type of apparatus is unsatisfactory because the hair employed repeatedly contracts and extends until in a short time it loses its elasticity and its calibration changes.

The dew point apparatus, on account of its complexities, is principally a laboratory apparatus. It is not suitable for general use.

Attempts have been made to overcome the disadvantages of the older types of apparatus by means of electrical hygrometers. These devices generally comprise a resistor that absorbs or adsorbs moisture and thereby changes in electrical conductivity. Theoretically the resistor absorbs moisture or gives it up to the atmosphere, and thereby decreases or increases in resistance, as the relative humidity of the atmosphere increases or decreases. Therefore, it is expected that by measuring the resistance of the resistor the relative humidity of the atmosphere can be determined.

Such electrical devices have not heretofore solved the problem because no satisfactory form of resistor has been available. The resistors heretofore employed have not been capable of quickly absorbing and giving up moisture directly in proportion to the moisture contained in the surrounding atmosphere and/or have not been of a sturdy enough construction to withstand prolonged usage and/or have deteriorated with use so as to require frequent replacement and/ or have not been practical for outdoor use.

I have discovered that a resistor comprising a homogeneous mixture of plaster of Paris and glycerine is free from the above stated defects and is otherwise suitable for the very accurate measurement of relative humidity when used as a part of an electric hygrometer. Accordingly, it is the principal object of this invention to provide a resistor for use with an electric hygrometer which comprises a substantially homogeneous mixture of plaster of Paris and a hygroscopic material such as glycerine.

Other objects and advantages of the invention will appear from the following description when read in connection with the accompanying drawing, wherein:

Figure 1 is a bottom plan view of a typical form of my improved resistor;

Figure 2 is a cross-sectional view taken on substantially the line 2--2 of Figure l; and

Figure 3 is a cross-sectional view of a portion of an electrical hygrometer employing my invention.

The device illustrated in the drawing includes: a plate I0 of insulating material such. as glass or plastic, provided with a recess I I on the lower face thereof, a pair of platinum wire loops I2 and I3 encircling the plate endwise and having portions thereof disposed within the recess; a thin layer of a, mixture of plaster of Paris and glycerine disposed within the recess II of plate In and bridging the space between the loops of wire I2 and I3; a pair of electrical connectors I4 and I5 connected respectively to the platinum wire loops I2 and I3; and an inverted glass funnel or flask I6 enclosing the plate I0 and associated elements.

The electrical connectors -I4 and I5 are adapted to be connected to any suitable electrical resistance measuring device such as a. Wheatstone bridge or the like, which is not shown. Generally speaking, practical resistance measuring devices do not measure resistance with ease, at a range greater than 5,000,000 ohms. Therefore, it is desirable to design and proportion the resistor so as to provide for a range of resistances which will not exceed that figure.

The essence of the present invention resides in the mixture of plaster of Paris and glycerine which constitutes the resistor per se. The proportions of these materials which are to be used will depend upon the use to which the instrument is to be put. For example, if a relatively large proportion of glycerine is used the resistance is decreased. If a relatively small proportion of glycerin@ is used the resistance is increased.

In viewy of the foregoing, it will be understood that the present invention lends itself to many different specic embodiments. Accordingly, the details of only one specific operative embodiment are herein shown and described. Of course, the

invention is 4not limited to that particular embodiment.

The plate Il is a plastic such as Lucite or Styron." It is V4 inch wide, 1 inch long, and Vs inch thick. In one iace of the plate and extending longitudinally thereof is a groove or trench II. The trench Il has a at bottom and vertical parallel side walls. The trench is l/s inch wide and 11n inch deep and extends throughout the length ot the plate I 0.

Two platinum wires I2 and I3 of No. 28 gauge are extended completely around the glass plate and the ends o1' each are soldered together to hold them in place. The wires lie in the trench II in spaced parallel relation to each other and con tact the sides of the trench. The platinum wires I2 and I3 are connected to insulated wire leads I4 and I5 respectively. Lead Il is connected to wire I2 at one end of the plate I0, while lead Il is connected to wire I3 at the other end of the plate, so that the resistor may be suspended within a suitable housing. The leads Il and Il extend away from the grooved face of the plate III so that when arranged as shown in Figure 3 the grooved face is downward.

Alter the wires I2, I3, I4 and I5 have been applied and properly insulated, the grooved face of the glass plate I is held face up and a small quantity of freshly made substantially homogeneous mixture of plaster of Paris and a water solution of glycerine is poured into the trench. The mixture comprises grams of chemically pure plaster o1' Paris and 10 cubic centimeters of a glycerine solution. Just enough of the mixture to cover the wires I2 and I3 is poured into the trench II and is spread inl a thin layer therein so as to bridge the space between the wires. The quantity needed will be less than 116 c. c. The mixture is permitted to set, which takes about 5 minutes, and is then cured by heating for several hours in an oven at approximately l F. This results in a very hard but porous solid Il. It has been found that plaster of Paris sets harder in a glycerine-water solution than in plain water. Thus, the resistor is tough, rugged and durable.

The mixture of plaster oi' Paris and glycerine has the power of absorbing moisture from the surrounding atmosphere or o1' giving it up to the atmosphere so that the moisture in the mixture is always in equilibrium with the moisture in the atmosphere. The electrical resistance of the mixture will vary as the moisture content oi' the surrounding atmosphere varies.

In the case of the resistor having the proportions described above and at a temperature of 70 F., the resistance varies from about 2,000 ohms at 100% to approximately 4,000,000 ohms at 10% relative humidity. To measure the relative humidity oi' the atmosphere, the resistance of the mixture is measured by causing a current to flow across the mixture between the platinum wires I2 and I3 and measuring the resistance of the mixture. In this manner, the relative humidity can be accurately determined, even at temperatures below 32 and above 100 F.

The resistor may be calibrated by subjecting it to various known vapor saturations created in stirred air in a closed space by distilled water or super saturated solutions of such salts as lithium chloride, magnesium chloride, magnesium nitrate, sodium chloride and potassium nitrate, at deilnite temperatures. The resistor may be calibrated also against a wet and dry bulb thermometer or a dew point measuring apparatus. As the electrical resistance is affected by temperature. proper correction is made in accordance with the existing temperature. This can be done by preparing suitable tables from test data obtained by noting resistance at constant humidities and variable temperatures.

In order to protect the resistor against rain, snow, wind, dust and water condensation, it is inserted into the inverted glass funnel or flask I8, as shown in Figure 3. The grooved face oi the glass plate I0 faces downward and the leads Il and I5 are brought out through the neck of the funnel. The open end of the funnel is plugged to prevent the entrance o! rain, snow and dust. The funnel may be suspended in inverted position by any suitable means,

The invention has proven extremely dependable, practical and serviceable. It is highly sensitive and accurate. It will function in all ordinary conditions, and can be employed under extraordinary conditions, such as extremely high temperatures and low humidities.

The invention can be placed in the inverted tunnel and kept outdoors in shelters such as those used by the U. S. Weather Bureau for housing recording instruments, and its accuracy will not be impaired, even at extremely low temperatures. Its calibration is permanent.

As indicated hereinbefore, the practical range of operation oi the device can be varied by varying the proportion of glycerine in the mixture. For example, if it is desired to measure extremely low humidities, the proportion of glycerine may be increased.

Likewise, the width of the trench and the consequent spacing of the wires I2 and Il may be varied as another way oi changing the resistance to change the range o! operation or to meet extraordinary conditions.

From the foregoing, it will be apparent that the present invention provides a resistor which is extremely sensitive and accuratethe range of measurement being from 4,000,000 ohms at 10% relative humidity to 2,000 ohms at 100% relative humidity, according to one embodiment. In all embodiments, resistor will be durable and of a permanent nature due to its composition. Likewise, I have discovered that this particular composition has the capacity to correctly reilect the humidity condition of the surrounding atmosphere.

The scope oi' the invention is indicated in the appended claims.

I claim:

1. An electrical resistance element comprising a thin layer of a homogeneous mixture of plaster of Paris and glycerine.

2. An electrical resistance element comprising a homogeneous mixture of plaster of Paris and a hygroscopic polyhydric alcohol.

3. An electrical resistance element comprising a solid resulting from the mixture oi equal parts of plaster of Paris and a solution oi' a hygroscopic polyhydric alcohol and water.

4. An electrical resistance element comprising a solid resulting from the mixture oi equal parts of plaster of Paris and a 15% glycerine solution in water.

5. An electrical resistance element comprising a non-conducting base, a thin layer of a homogeneous mixture of plaster oi Paris and a hygroscopic polyhydric alcohol thereon, and a pair oi spaced apart electrical conductors in conductive relation with said layer.

6. An electrical resistance element comprising asoman 3 with both of said conductors. v

7. An electrical resistor comprising a at strip of insulating material having a channel shaped trench in one face thereof, a pair of spaced apart parallel conductors in said trench, and a thin layer: of a homogeneous mixture of plaster of Paris and glycerine in said trench. said mixture being in contact with both oi said conductors.

8. An electrical resistance element comprising a non-conducting base having a shalloyf'trench in one face thereof, a pair of spaced apart parallel electrical conductors in said trench, and a thin layer of a homogeneous mixture of lplaster of Paris and a hygroscopic polyhydric alcohol in said trench. said mixture being in electrical con- 10 tact with both of said conductors.

GEORGE BOUYOU-COS. 

